Page:Popular Science Monthly Volume 2.djvu/314

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300
THE POPULAR SCIENCE MONTHLY.

color in the sun. The tints of fruits in the same way develop under the healthy action of daylight, and the rule extends to those principles of every nature which give taste and odor to the different parts of the the plant.

Flowers, fruits, and leaves, then, are elaborated by the help of luminous vibrations. Their tissue holds the sun's rays. Those charming colors, those fragrant perfumes, and delicious flavors, all the innocent pleasures the vegetable kingdom yields us, owe their creation to light. The subtle working of these wonderful operations eludes us, as does that which guides the fleeting diffusion and thousand-fold refractions displayed by the imposing spectacle of the dawn; but is it nothing to gain a glimpse of those primal laws, and to possess even a twilight ray upon these magnificent phenomena?

 
II.

Light exerts a mechanical influence on vegetables. The sleep of flowers, the bending of their stems, the nutation of heliotropic plants, the inter-cellular movements of chlorophyll, offer proofs of an extremely delicate sensitiveness of certain plants in this respect. Pliny mentions the plant called the sunflower, which always looks toward the sun, and steadily follows its motion. He notices, too, that the lupin always follows the sun in its daily movement, and points out the hour for laborers. Tessier, at the end of the last century, took up the study of these phenomena, and inferred in a general way that the stems of plants always turn toward the light, and bend over, if necessary, to receive it. He noted, too, that leaves tend to turn toward the side whence daylight comes. Payer made more exact experiments. He tried them with young stems of common garden cresses grown on damp cotton in the dark. These stems have the property of curving and turning quickly when placed in a room lighted only from one side or in a box receiving light on one wall only. The upper part of the stem curves first, the lower part remaining straight. By a second movement the top erects and the bottom bends over, so that the plant, though leaning, becomes almost rectilinear again. When put in a room receiving light from two windows, the following results are noticed: If the openings are on the same side admitting light equally, the stem bends in the direction of the middle of the angle formed by these two beams; if one of the two windows admits more light than the other, the stem leans toward it; if the windows are opposite each other, the stem stands erect, when light comes equally from both sides, and, if it does not, turns toward the stronger rays. Payer discovered, moreover, that the part of the irradiating light most active in its effects corresponds in this case to the violet and the blue. The red, orange, yellow, and green rays, do not seem to produce any movement in plants. Gardner carried the investigation still further. He sowed turnips, and let them develop in the dark to two or three inches in